Flexible engine mountings support the engine and gearbox assembly while reducing the transmission of vibration and noise to the structure of the vessel.
They are often selected solely according to the total weight of the engine. In reality, correct selection also requires consideration of load distribution, centre of gravity, transmitted torque, propeller thrust, type of application and installation configuration.
A mounting that is too soft may allow excessive engine movement. A mounting that is too rigid, however, may transmit more vibration to the hull and reduce comfort on board.
This guide explains the function of R&D Marine engine mountings, the information required to select them and the main checks to carry out during installation and maintenance.
What Are R&D Marine Engine Mountings?
Flexible engine mountings are installed between the engine feet and the vessel’s engine beds.
They must perform several functions at the same time:
- support the weight of the engine and gearbox assembly;
- reduce the transmission of vibration;
- limit structure-borne noise;
- permit small, controlled movements;
- retain the engine in the intended position;
- contribute to correct alignment with the propeller shaft;
- withstand reactions generated by torque and thrust.
R&D Marine mountings are available in different configurations. Depending on the installation, shear-type mountings may provide effective vibration isolation, while compression-type mountings may be more suitable where the available space is limited.
Slotted fixing holes and adjustable studs also help with positioning and alignment.
A flexible mounting does more than support the engine: it must control engine movement without eliminating vibration isolation.
Why Is the Engine Installed on Flexible Mountings?
marine engine generates vibration as a result of:
- combustion;
- rotating and reciprocating masses;
- cyclic torque irregularities;
- gearbox operation;
- propeller-shaft rotation;
- propeller operation;
- changes in load.
If the engine were fixed rigidly to the engine beds, a greater proportion of this vibration would be transmitted directly to the vessel’s structure.
The flexible material in the mounting deforms in a controlled manner and reduces the amount of vibrational energy transferred to the hull.
Isolation must not, however, be excessive. The engine must be allowed to make small movements, while remaining sufficiently restrained to avoid excessive loads on the gearbox, flexible coupling, propeller shaft, hoses and other connections.
The Mountings Do Not All Carry the Same Load
One of the most common mistakes is to divide the total weight of the engine and gearbox assembly by four.
This calculation provides only an average theoretical load.
The actual load carried by each mounting depends on:
- the position of the centre of gravity;
- gearbox weight;
- engine length;
- distance between the front and rear mountings;
- position of the engine feet;
- installation angle;
- fitted accessories;
- distribution of components around the engine.
The rear mountings, close to the gearbox, may carry a different load from the front mountings. Some installations may therefore require mountings with different load capacities at the front and rear.
The total engine weight must not automatically be divided into equal parts without considering the actual load distribution.
How to Estimate the Load on Each Mounting
For an accurate assessment, the position of the centre of gravity of the complete engine and gearbox assembly should be known in relation to the mounting points.
When this information is unavailable, selection may be based on:
- the engine manufacturer’s recommendations;
- application tables;
- information from the existing installation;
- direct load measurements;
- experience with equivalent engines and gearboxes.
The complete operating weight should be considered, including:
- engine;
- gearbox;
- lubricating oil;
- coolant;
- accessories supported directly by the engine;
- brackets and attached components.
The correct mounting depends not only on engine dimensions and weight, but also on vibration characteristics and operating conditions.
Effect of Torque on the Mountings
When an engine transmits torque, the engine block tends to react in the opposite direction to crankshaft rotation.
This reaction temporarily changes the load on the mountings:
- some mountings are compressed more heavily;
- others are partially unloaded;
- the engine assembly may tilt slightly;
- alignment may change during acceleration.
Torque must therefore not be considered only when selecting the damper plate or flexible coupling. It also affects the behaviour of the engine mountings.
This effect is particularly important with:
- high-torque engines;
- low operating speeds;
- high gearbox reduction ratios;
- propellers with high rotational inertia;
- rapid acceleration;
- commercial applications;
- frequent manoeuvring.
Effect of Ahead and Astern Gear
When gear is engaged, the propeller generates longitudinal thrust.
Depending on the installation, this thrust may be transferred through the propeller shaft, flexible coupling and gearbox to the engine assembly.
An engine installed on flexible mountings may therefore tend to move:
- in one direction when ahead gear is engaged;
- in the opposite direction when astern gear is engaged.
This movement temporarily changes the position of the gearbox flange in relation to the propeller shaft.
The mountings must be sufficiently flexible to isolate vibration, but they must also control these longitudinal movements.
Engine movement when gear is engaged depends not only on mounting softness, but also on thrust, torque, alignment and the condition of the complete propulsion system.
Mountings That Are Too Soft
A mounting with insufficient capacity, or one that is excessively soft, may deform too much under static and dynamic loads.
Possible consequences include:
- excessive engine movement;
- significant changes in alignment;
- vibration during acceleration;
- contact between the engine and surrounding components;
- stress on hoses and the exhaust system;
- overloading of the flexible coupling;
- wear of propeller-shaft seals;
- permanent deformation of the mounting;
- failure of the rubber element.
A very soft mounting may provide good isolation under certain conditions, but it is not suitable if it cannot keep engine movement within the permitted limits.
Mountings That Are Too Rigid
A mounting that is too rigid restricts engine movement but reduces its ability to isolate vibration.
Possible consequences include:
- increased structure-borne noise;
- vibration transmitted through the hull;
- resonance at particular engine speeds;
- stress on the engine feet;
- increased transmission of torque impulses;
- reduced comfort on board.
Correct selection therefore requires a balance between:
- load capacity;
- stiffness;
- static deflection;
- control of movement;
- vibration isolation.
The stiffest mounting is not necessarily the safest, just as the softest mounting does not necessarily provide the best isolation in every installation
.Shear-Type and Compression-Type Mountings
In a compression-type mounting, the engine weight acts mainly by compressing the flexible element.
This arrangement can be advantageous when:
- vertical or lateral space is limited;
- a compact design is required;
- the installation requires a particular mounting geometry.
In a shear-type mounting, the elastomer deforms in a different manner and may provide effective vibration isolation.
The choice between these arrangements must not be based solely on their appearance. It is necessary to check:
- permitted load;
- overall height;
- adjustment range;
- stiffness;
- available space;
- position of the fixing holes;
- type of application.
R&D Marine offers different configurations to suit different installation requirements.
Importance of Height and Adjustment
The threaded studs allow the engine height to be adjusted so that the gearbox can be aligned with the propeller shaft.
This adjustment must be carried out correctly.
A stud extended too far may:
- increase leverage;
- reduce stability;
- encourage oscillation;
- subject the mounting to abnormal lateral loads;
- make correct tightening more difficult.
A stud set too low may leave insufficient space for adjustment and for the locking nuts.
Mountings must not be used beyond their intended adjustment range.
A Mounting Must Not Correct a Structural Error
Mounting adjustment is intended to position the engine correctly. It must not be used to compensate for:
- distorted engine beds;
- engine feet that are not level;
- bent brackets;
- an incorrectly positioned propeller shaft;
- mountings installed on uneven surfaces;
- excessive differences in height.
When one or more mountings must be adjusted close to their limit in order to obtain alignment, the geometry of the installation should be checked.
A flexible mounting must not be used to conceal a fault in the engine beds or propeller-shaft alignment.
Relationship with the Flexible Coupling
Engine mountings and the flexible coupling work together, but they perform different functions.
The mountings:
- support and position the engine;
- isolate vibration;
- control movement of the engine and gearbox assembly.
The flexible coupling:
- connects the gearbox to the propeller shaft;
- reduces rigidity between the flanges;
- accommodates small relative movements;
- limits the transmission of vibration and shocks.
A flexible coupling cannot compensate for deteriorated or incorrectly selected engine mountings.
Likewise, new mountings cannot correct major misalignment between the gearbox and propeller shaft.
Alignment After Replacing the Mountings
Alignment must always be checked after replacing the engine mountings.
Even when the new mountings appear to have identical dimensions, they may differ in:
- loaded height;
- stiffness;
- static deflection;
- stud position;
- manufacturing tolerances;
- actual load distribution.
Alignment should be checked:
- after installation;
- after the initial settling period;
- after several hours of operation;
- after launching, where applicable;
- after work on the propeller shaft;
- whenever new vibration appears.
The slotted holes and adjustable studs used on R&D Marine mountings help with positioning and alignment.
Tightening and Locking Nuts
After adjustment, all fasteners must be tightened correctly.
Check:
- mounting base bolts;
- nuts on the threaded studs;
- locking nuts;
- washers;
- full contact with the engine bed;
- absence of looseness;
- absence of lateral stress.
The nuts must not be tightened in a manner that improperly locks or deforms the flexible element.
The mounting must remain able to operate in the configuration intended by the manufacturer.
Contamination by Oil, Fuel and Other Fluids
Oil or fuel around the mountings should always be investigated.
Even when the elastomer is designed to withstand the marine environment, continuous contamination may:
- alter the rubber;
- cause swelling;
- reduce stiffness;
- promote separation between materials;
- retain dirt;
- conceal cracks and corrosion.
The cause of the leak should therefore be corrected, and the mountings should be cleaned and inspected periodically.
Signs of Worn Engine Mountings
The main warning signs include:
- cracked rubber;
- hardened rubber;
- swelling;
- permanent collapse;
- separation between rubber and metal;
- a tilted mounting;
- differences in height;
- corrosion;
- a bent stud;
- a loose nut;
- excessive engine movement;
- increased vibration;
- shocks when gear is engaged;
- abnormal movement of hoses or the exhaust system;
- alignment that changes repeatedly.
A mounting may be deteriorated even when it has not completely failed.
Why One Mounting Should Not Be Replaced Without Further Checks
When only one mounting appears damaged, the remaining mountings must also be inspected.
The failure may have been caused by:
- incorrect load distribution;
- an unsuitable mounting;
- misalignment;
- an uneven engine bed;
- insufficient tightening;
- contamination;
- excessive engine movement;
- a difference in stiffness compared with the other mountings.
Replacing one mounting with a model of different stiffness may further alter the load distribution.
It is not always necessary to replace all the mountings, but the complete system should be assessed.
Information Required to Select R&D Marine Engine Mountings
To identify the correct mountings, it is useful to provide:
- engine manufacturer and model;
- gearbox manufacturer and model;
- engine weight;
- gearbox weight;
- total assembly weight;
- number of mountings;
- mounting positions;
- distance between the front and rear mountings;
- approximate centre-of-gravity position;
- engine power;
- maximum engine speed;
- torque;
- gearbox reduction ratio;
- type of application;
- presence or absence of a separate thrust bearing;
- required mounting height;
- stud diameter;
- fixing-hole spacing and dimensions;
- available space;
- model of the existing mountings.
Photographs of the installation and measurements of the existing mountings are also very useful.
Checks Before Installation
Before fitting the mountings, check:
- compatibility with the expected load;
- mounting dimensions;
- height and adjustment range;
- stud diameter;
- fixing-hole position;
- flatness of the engine beds;
- condition of the engine feet;
- absence of corrosion;
- access to the nuts;
- clearance for engine movement;
- condition of the flexible coupling;
- ability to carry out correct alignment.
The engine beds must be sufficiently rigid and provide an adequate supporting surface.
Checks After Installation
After installation, check:
- engine height;
- visible load distribution;
- tightness of the fasteners;
- stud position;
- flange alignment;
- freedom of movement of hoses and cables;
- clearance around the engine;
- behaviour at idle;
- vibration at different speeds;
- movement when ahead and astern gear are engaged;
- unusual noise or contact.
After an initial period of operation, it is advisable to check again:
- settling;
- fastener tightness;
- height;
- alignment;
- any abnormal collapse.
Common Mistakes
The most common mistakes include:
- selecting the mountings by dividing the total weight by four;
- ignoring the position of the centre of gravity;
- fitting four mountings with unsuitable load capacity;
- selecting mountings that are too soft;
- selecting mountings that are too rigid;
- failing to consider torque and thrust;
- using studs that are almost fully extended;
- installing mountings on uneven surfaces;
- forcing the engine into position;
- failing to tighten the locking nuts;
- replacing one mounting without checking the others;
- failing to check alignment after replacement;
- ignoring oil or fuel leaks;
- automatically blaming every vibration problem on the mountings.
Conclusion
R&D Marine engine mountings are not simply rubber supports.
They must support the engine and gearbox assembly, isolate vibration, control movement generated by torque and thrust, and retain the engine in the position required for correct propeller-shaft operation.
Selection must not be based solely on total engine weight. It is also necessary to consider:
- load distribution;
- centre-of-gravity position;
- torque;
- thrust;
- type of application;
- installation characteristics;
- mounting height and fixing geometry.
Correctly selected engine mountings, accurate alignment and a suitable flexible coupling work together to reduce vibration and stress throughout the propulsion system.